DESIGN FEATURES L Internal 2A, 42V Switch, Adjustable 2.5MHz Operating Frequency and 3mm × 3mm Package Allow Boost Regulator to Fit Numerous Applications by Mathew Wich Introduction The world of switching DC/DC converters is awash with a dizzying array of product offerings. For a given application, much of the power supply design effort can be spent simply searching for the optimum combination of package size, switching frequency, input and output voltage range, and desirable features. In many cases, though, the LT3580 offers an optimal solution. It is the right choice for many diverse applications because of its smart combination of features, performance and ease of use. The LT3580 is a current control switching regulator available in The LT3580 supports a variety of converter configurations including boost, inverting, flyback, and SEPIC. Inputs can be from 2.5V–32V, and an integrated 2A, 42V NPN power switch allows the LT3580 to provide efficient power from a fraction of a watt up to more than several watts. L1 4.2µH VIN 5V VIN SHDN RT C2 10µF SW GND LT3580 SYNC C1 2.2µF D1 75k FB SS VOUT 12V 550mA 130k VC 10k 0.1µF 1nF C1: 2.2µF, 25V, X5R, 1206 C2: 10µF, 25V, X5R, 1206 D1: MICROSEMI UPS120 L1: SUMIDA CDR6D23MN-4R2 95 1200 90 1000 80 800 75 600 70 65 400 60 200 55 50 POWER LOSS (mW) EFFICIENCY (%) 85 0 100 200 300 400 LOAD CURRENT (mA) 500 600 0 Figure 1. This 1.2MHz, 5V to 12V boost converter achieves over 88% efficiency. Linear Technology Magazine • January 2008 tiny 8-lead packages (MSOP and 3mm × 3mm DFN). Operating from 200kHz–2.5MHz, it supports numerous configurations including boost, inverting, flyback and SEPIC. Inputs can be from 2.5V–32V, and an integrated 2A, 42V NPN power switch allows the LT3580 to provide efficient power from a fraction of a watt up to more than several watts. Be Picky—Choose the Ideal Clock Frequency up to 2.5MHz Choosing a converter switching frequency is often a compromise between several performance parameters such as physical size, output ripple, efficiency and spectral noise issues. While most converter ICs operate at a single fixed frequency, the LT3580 operates at any frequency from 200kHz–2.5MHz allowing you to choose the ideal frequency for any application. The high frequency capability (up to 2.5MHz) of the LT3580 helps to reduce the overall size of the converter by permitting the use of smaller inductors and output capacitors. Small inductors, with correspondingly small inductances, work best at higher frequencies because they store and release less energy in each switching cycle. This can be seen by looking at the energy storage relationship for an inductor, E= 1 2 LI , 2 which shows that for a given peak inductor current (I), the stored energy is proportional to the inductance (L). Thus smaller inductances, storing less energy per cycle, switch at L DESIGN FEATURES RC CSS 7 SHDN – + 5 1.3V VIN CC 2 SS CIN VC DISCHARGE DETECT L1 275k UVLO SR2 R 3 ILIMIT COMPARATOR – Q2 Q VIN S 1.215V REFERENCE A3 + R 4 SR1 DRIVER S VOUT C1 Q1 Q RFB + + 14.6k Σ A1 – FB D1 SW VC SOFTSTART A4 RAMP GENERATOR 1 + 14.6k FREQUENCY FOLDBACK A2 0.01Ω – GND 9 ÷N ADJUSTABLE OSCILLATOR – SYNC BLOCK SYNC 8 LT3580 RT 6 RT Figure 2. Block diagram of the LT3580 in a boost converter configuration V=L di ΔI ⇒L dt ΔT and solving for ΔT. ΔT = L • ΔI V This shows that, for a given inductor voltage (V), a smaller inductor (L) will ramp to its peak current (I) in less L1 3.3µH VIN 5V D1 RT SW GND LT3580 SYNC C1 4.7µF C2 4.7µF FB SS 35.7k 130k VC 10k 0.1µF 2.2nF 47pF VOUT 12V 500mA solution in Figure 1. The tradeoff is slightly reduced efficiency due to the increased switching losses incurred at the higher switching frequency. For large voltage gains, the LT3580’s low frequency capability (down to 200kHz) is very useful. Figure 5 shows a direct conversion from 5V to 40V running at 750kHz. Figure 6 shows a 5V to 350V flyback converter running at 200kHz. Finally, the LT3580’s wide frequency range makes it easy to avoid sensitive frequency bands that can’t tolerate spectral noise. For example radio power supplies may operate 95 1400 90 1200 85 1000 80 75 800 70 600 65 400 60 200 55 C1, C2: 4.7µF, 25V, X5R, 1206 D1: MICROSEMI UPS120 L1: COILCRAFT LPS4018-332ML 50 POWER LOSS (W) VIN SHDN time (T) than a larger inductance, again leading to higher frequency operation to make best use of the inductor. Depending on the load requirements, high frequency operation also facilitates smaller output capacitors. Since charge is delivered to the output in smaller but more frequent packets, the voltage ripple is reduced for a given capacitance. Figure 3 shows an example of reduced solution size at a higher switching frequency. The 5V to 12V boost converter operates at 2.5MHz and uses a smaller inductor and less output capacitance than the 1.2MHz EFFICIENCY (%) higher frequencies to deliver the same power as larger inductances. Also, smaller inductances reach their peak current (or energy) faster than large inductances as seen by rearranging the relationship 0 100 200 300 400 LOAD CURRENT (mA) 500 0 600 Figure 3. The high 2.5MHz switching frequency of this 5V to 12V boost converter allows the use of a tiny 4mm × 4mm × 1.7mm inductor. Linear Technology Magazine • January 2008 DESIGN FEATURES L 2.7 FREQUENCY (MHz) 2.5 RT = 35.7k L1 47µH VIN 5V 2.3 VIN 2.1 SHDN 1.7 1.3 1.1 –50 C1 2.2µF FB VOUT 40V 150mA 464k VC SS 10k 121k 0.1µF 47pF 4.7nF RT = 75k 50 0 TEMPERATURE (°C) C1, C2: 2.2µF, 25V, X5R, 1206 D1: MICROSEMI UPS140 L1: SUMIDA CDRH105R-470 100 Figure 4. Typical internal oscillator frequency at VIN = 5V at 2MHz or above to avoid the AM broadcast band. Also, some RF communications products are sensitive to noise at 455kHz, therefore switching above 600kHz is desired. Accurate Clocking Options The LT3580 provides two options for generating the clock. First, the integrated oscillator can be accurately set between 200kHz–2.5MHz by connecting a single resistor from the R T pin to ground, where R T (kΩ) = GND LT3580 SYNC 1.5 C2 2.2µF SW RT 1.9 D1 91.9 −1 fOSC (MHz) The boost converter in Figure 3, for example, uses a 35.7k R T resistor to set the switching frequency to 2.5MHz. The internal oscillator’s frequency is accurate to ±10% with little temperature variation as shown in Figure 4. The excellent frequency tolerance maximizes system performance by reducing necessary design margin. The switching frequency can also be synchronized to an external clock source. The SYNC pin overrides the internal oscillator when toggled at frequencies greater than 75% of the internal oscillator’s set frequency. Simply connect a digital clock signal to the SYNC pin using VIH levels from 1.3V to 5.5V, VIL levels below 0.4V and any frequency between 200kHz and 2.5MHz. Using an external clock source is often helpful for several reasons, including… qSynchronization of several switching regulators, often out of phase, to reduce switching current spikes Linear Technology Magazine • January 2008 Figure 5. A 750kHz, 5V to 40V, 150mA boost converter qAdditional frequency precision yielding higher performance qPrecisely targeting the frequency out of sensitive bands for EMI benefits. The LTC6908 resistor set oscillator is a nice choice for generating the SYNC clock due to its high precision, dual phase outputs, spread spectrum capabilities, small size and simple operation. Single-Pin Feedback and Support for Multiple Configurations The novel single-pin feedback of the LT3580 reduces external component count and allows it to be used in many different converter topologies. The output voltage is set by simply connecting a single external resistor from VOUT to the FB pin. The FB pin automatically servos to the correct reference voltage for a given topology (1.215V for positive VOUT and 5mV for negative VOUT). Supported configurations include boost, SEPIC (Figure 10), and other topologies such as the flyback (Figure 6) and inverting (Figure 7). Finally, to improve VOUT accuracy, the FB pin is factory trimmed to an accurate current, instead of trimming the resistance, which is typical of other parts. This eliminates multiplication of reference voltage errors to VOUT. Soft-Start Feature Limits Start-Up Current The LT3580 contains a soft-start circuit to limit peak switch currents during start-up. High start-up current is inherent in switching regulators since the feedback loop is saturated Danger High Voltage! Operation by High Voltage Trained Personnel Only VIN 3.3V TO 5V T1 1:10.4 7, 8 • 4.7MH • 5, 6 VOUT 350V 4.5mA (VIN = 5V) 2.5mA (VIN = 3.3V) D1 1 4 C2 68nF D2 VIN SW GND SHDN RT LT3580 SYNC C1 2.2MF 464k FB SS FOR ANY VOUT BETWEEN 50V TO 350V, CHOOSE RFB ACCORDING TO RFB 4.22M* V – 1.215 RFB = OUT 83.3MA VC 10k 0.47MF 100pF 10nF C1: 2.2MF, 25V, X5R, 1206 C2: TDK C3225X7R2J683M D1: VISHAY GSD2004S DUAL DIODE CONNECTED IN SERIES D2: ON SEMICONDUCTOR MBR0540 T1: TDK LDT565630T-041 FOR 5V INPUT, KEEP MAXIMUM OUTPUT POWER AT 1.58W FOR 3.3V INPUT, KEEP MAXIMUM OUTPUT POWER AT 0.88W *MAY REQUIRE MULTIPLE SERIES RESISTORS TO COMPLY WITH MAXIMUM VOLTAGE RATINGS Figure 6. This 350V power supply features a tiny 5.8mm × 5.8mm × 3mm transformer switching at 200kHz. L DESIGN FEATURES due to VOUT being far from its final value. The regulator tries to charge the output capacitors as quickly as possible, which results in large peak currents. The start-up current can be limited by connecting an external capacitor (typically 100nF to 1µF) to the SS pin. This capacitor is slowly charged to ~2.2V by an internal 275k resistor once the part is activated. SS voltages below ~1.1V reduce the internal current limit. Thus, the gradual ramping of SS also gradually increases the current limit as the capacitor charges. This, in turn, allows the VOUT capacitor to charge gradually toward its final value while limiting the start-up current (see Figure 9). VIN 3.3V TO 12V VIN SYNC continued on page 28 10 VOUT –5V 800mA (VIN = 12V) C2 620mA (VIN = 5V) 10µF 450mA (VIN = 3.3V) D1 GND LT3580 RT C1 2.2µF L2 4.7µH SW SHDN 60.2k FB VC SS 35.7k 10k 100pF 0.1µF 2.2nF C1: 2.2µF, 25V, X5R, 1206 C2: 10µF, 25V, X5R, 1206 C3: 1µF, 50V, X5R, 0805 D1: CENTRAL SEMI CMMSH1-40 L1, L2: COILCRAFT LSP4018-472ML Figure 7. This –5V output inverting converter switches at 2.5MHz and accepts inputs between 3.3V and 12V VIN VIN – 1.3V RUVLO1 Innovative SHDN Pin Resets Soft-Start and Serves as Undervoltage Lockout (UVLO) The SHDN pin has threshold hysteresis to resist noise and tolerate slowly varying input voltages. Driving the SHDN pin to ground shuts down the LT3580 and reduces input current to less than 1µA. Driving SHDN above 1.38V enables the part and begins the soft-start sequence. A built in safety feature ensures that the SS capacitor is actively discharged before start-up begins. This allows for proper soft-start even in the event of short SHDN pulses or thermal lockout. The LT3580 also features an integrated UVLO that shuts down the chip when the input voltage falls below ~2.3V. However, the SHDN pin can also be configured to disable the chip below even higher voltages as shown in Figure 8. Typically, UVLO is needed in situations where the input supply is current-limited, has a relatively high source resistance, or ramps up/down slowly. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current-limit or latch low under low voltage conditions. The configurable UVLO prevents the regulator from operating at source C3 1µF L1 4.7µH SHDN ACTIVE/ LOCKOUT + 11.6µA AT 1.3V RUVLO2 (OPTIONAL) GND LT3580 Figure 8. Configurable undervoltage lockout SHDN 2V/DIV SS 0.5V/DIV VOUT 5V/DIV IL 500mA/DIV VIN = 5V VOUT = 12V 2ms/DIV Figure 9. Soft-start of a 5V to 12V boost topology C3 1µF L1 4.7µH VIN 2.6V TO 12V OPERATING 12V TO 32V TRANSIENT VIN SHDN RT GND SYNC C1 2.2µF FB SS 35.7k VOUT 5V, 600mA (VIN = 5V OR HIGHER) 500mA (VIN = 4V) C2 400mA (VIN = 3V) 10µF 300mA (VIN = 2.6V) L2 4.7µH SW LT3580 D1 46.4k VC 10k 0.1µF 22pF 1nF C1: 2.2µF, 35V, X5R, 1206 C2: 10µF, 10V, X5R, 1206 C3: 1µF, 50V, X5R, 0805 D1: MICROSEMI UPS140 L1, L2: TDK VLCF4020T-4R7N1R2 Figure 10. Wide input range SEPIC converter with 5V output switches at 2.5MHz Linear Technology Magazine • January 2008 L DESIGN IDEAS with the flexibility to program both the upper and lower battery temperature points that are considered safe for charging the battery. In this example, the NTC hot and cold trip points are set for approximately 55°C and 0°C, respectively. One of the buck regulators is programmed for 3.3V at its output. When the BAT pin voltage approaches 3.3V, the buck regulator operates in dropout. An LED at the CHRG pin gives a visual indication of the battery charge status. Figure 2 shows an actual circuit similar to that shown in Figure 1, illustrating how little board space is required to build a full featured LTC3559 application. Figure 3 shows how much more efficient Burst Mode operation is at light loads as compared to pulse skipping mode. A basic sequencer function can be built for the buck regulator outputs by driving the enable pin on one buck Figure 4 helps to explain this scenario. The current being delivered at the BAT pin is 500mA. Both buck regulators are enabled. The sum of the average input currents being drawn by both buck regulators is 200mA. This makes the effective battery charging current only 300mA. If the HPWR pin were tied low, the BAT pin current would be only 100mA. With the buck regulator conditions unchanged, this would cause the battery to discharge at 100mA. Conclusion Figure 2. A USB battery charger and two buck regulators small enough to fit in the latest cell phones, PDAs and MP3 players regulator with the output of the other buck regulator. For proper operation, the BAT and PVIN pins must be tied together. If a buck regulator is enabled while the battery is charging, the net current charging the battery will be lower than the actual programmed value. 500mA 100 80 EFFICIENCY (%) USB (5V) Burst Mode OPERATION 90 VCC 60 RPROG 1.62k PULSE SKIP MODE 50 SUSP 40 HIGH 30 HIGH VOUT = 1.2V PVIN = 2.7V PVIN = 4.2V 10 0 0.1 1 10 ILOAD (mA) 100 HIGH LOW (PULSE SKIP MODE) 300mA BAT PVIN PROG 70 20 + SINGLE Li-lon CELL 3.6V 200mA + 2.2µF LTC3559 HPWR SW1 VOUT1 EN1 SW2 VOUT2 EN2 MODE 1000 Figure 4. The net current charging the battery depends on the operating mode of the buck regulators. Figure 3. Buck regulator efficiency LT3580, continued from page 10 voltages where these problems might occur. The shutdown pin comparator has voltage hysteresis with typical thresholds of 1.32V (rising) and 1.29V (falling). Resistor RUVLO2 is optional but can be included to reduce overall UVLO voltage variation caused by variations in SHDN pin current. A good choice for RUVLO2 is 10k ±1%. After choosing a value for RUVLO2, RUVLO1 can be determined from either of the following: 28 The LTC3559 is ideally suited for space-constrained applications that are powered from a single Li-Ion cell and that need multiple voltage supply rails. The high switching frequency allows the use of small low profile external inductors. The high efficiency buck regulators and Burst Mode operation combine to maximize battery life, extending battery operation time between charge cycles. L RUVLO1 = VIN − 1.32V 1.32V R + 11.6µA UVLO2 + or RUVLO1 = VIN − − 1.29 V 1.29 V R + 11.6µA UVLO2 where VIN+ and VIN- are the VIN voltages when rising or falling respectively. Conclusion The LT3580 is a smart choice for many DC/DC converter applications. It’s packed with features without compromising performance or ease of use and is available in tiny 8-lead packages. The accurate and adjustable clock, 2A/42V power switch, wide input voltage range, integrated soft-start and a configurable SHDN pin make the LT3580 an ideal choice for many DC power supply needs. For additional information and a complete data sheet visit www.linear.com. L Linear Technology Magazine • January 2008